[Serusers] Different behaviour from different clients, same firewall.

What are some things that might create one-way audio issues from different clients, all using STUN, all NATted behind the same firewall? Some UAs, I have to add port-forwarding for to ensure two-way audio (they're behaving as though they're behind a symmetric NAT). Others, I don't have any issues with at all. For instance, X-Lite? Always works. Snom? Requires port-forwarding in order to function correctly 100% of the time (often works, doesn't always). Grandstream (phones and ATAs)? Always works. UTStarCom F3000? Requires port-forwarding to function correctly. SJPhone? Always works. Etc., etc. All these devices are behind the same firewall, so I don't think it's necessarily an issue with the method of NAT that the firewall does. It's something that varies inside the UA. Any ideas what, inside a UA, could cause such disparity? I thought it might be the SDP headers, but I can't honestly tell MUCH difference between them. I'm at a bit of a loss for all the bizarre variables involved. N.